Why were Americans susceptible to ideas like McCarthyism?

The early years of the Cold War were characterized by hysteria and fear of Communism. While these fears may seem irrational and unfounded to modern Americans, they should be understood in their context. By 1949, a number of troubling developments had occurred. The Soviet Union had expanded its influence over all of Eastern Europe, and it was by no means certain that war over the fate of Berlin would not soon occur. Mao Zedong, a communist, led a takeover of China, which, from an American perspective, meant the world's most populous nation had fallen to Communism.

In 1950, American fears, it seemed, were realized when communist North Korea invaded the South, leading to US-led intervention on the peninsula. Additionally, Americans were regularly besieged with government propaganda about the evils of communism. Investigations first at the state level and then by the House Un-American Activities Committee (HUAC) revealed that many individuals in government had ties to the Communist Party (which was not at all uncommon in the 1930s.) Alger Hiss, a high-ranking State Department official, was accused of being a Soviet agent. It seemed to some Americans (and the media did not dissuade them from holding this notion) that communism was on the march, and that a fifth column of Communists existed within the United States, ready to engineer a revolution.

It was within this charged environment that Joseph McCarthy cynically used accusations of communist allegiances to damage others' political standing and enhance his own. People found him believable because he played on the deepest fears of many Americans.